The present invention relates to an improvement in the method for the preparation of a laminated inductor device. More particularly, the invention relates to an improvement in the method for the preparation of a surface-mountable laminated inductor device of high performance for accomplishing higher quality of the products and higher efficiency of the preparation process.
As a general trend of the market demand, electronic instruments are desired to be more and more compact and the parts constituting the instrument also must be more and more compact accordingly.
Some of electronic parts such as inductors, capacitors and the like which are heretofore designed to be provided with lead terminals can now be more compact by the application of a recently developed technology in which ceramic layers in a specific patterned form and metallic layers consisting a laminated body are subjected to a simultaneous sintering treatment to form a monolithic structure provided with internal conductors.
As a trend in recent years, chip capacitors and chip resistors are under shift toward the 1005 form. Chip inductors are also desired to be in a more compact size.
In the preparation of a laminated inductor device, a powder of a ceramic material such as ferrites is first blended and slurried with a binder and an organic solvent to give a pasty mixture which is applied onto a substrate film such as a polyethylene terephthalate film and the like by using a doctor blade or other suitable coating means followed by drying to form a so-called ceramic green sheet. Thereafter, through-holes are formed to penetrate this ceramic green sheet by machining or laser working and a coil pattern is formed thereon by the method of screen printing with a silver- or silver/palladium-based electroconductive paste. In this case, electroconduction between two layers is obtained by the conductive paste filling the through-holes.
In the next place, a plurality of the thus printed ceramic green sheets are laid one on the other and bonded together by compression under heating to give a laminated block which is cut into a desired form of chips followed by removal of the binder and sintering. The thus sintered chips are ground and provided with electrode terminals followed by a further heat treatment and final electrolytic plating treatment to form a coating layer on the electrode terminals.
An example of the thus obtained chip inductors is illustrated in FIG. 1 by a perspective view showing the internal structure having a coil F, internal conductors M and through-holes S for electric conduction. FIG. 2 is a flow sheet diagram showing an example of the preparation process of such a chip inductor.
It is of course indispensable in the above described process for the preparation of chip inductors that the step of lamination of the ceramic green sheets is preceded by removal of the substrate film by peeling off the ceramic green sheet in each of the laminated bodies.
When the size of the thus prepared chip inductors is relatively large as is mostly the case heretofore, the cross section of the coil can be large enough so that, while a coil of five turns is usually sufficient, the thickness of the ceramic green sheet can be about 50 xcexcm at the smallest and no difficulty is encountered in the removal of the substrate film by peeling. When the chip size is greatly decreased to comply with the recent requirement as is the case in the 1005 form, for example, the cross sectional area of the coil is necessarily so limited that a desired inductance can be obtained only by increasing the number of turns of the coil.
As a consequence, the thickness of the ceramic green sheet must be decreased to be 20 xcexcm or even smaller. When a ceramic green sheet of such a small thickness is separated from the substrate film by peeling, troubles are sometimes unavoidable including breaking of the ceramic green sheet, formation of creases and stretching of the sheet.
In the laminated inductors, moreover, the above described troubles are even more remarkable because it is sometimes unavoidable that punching of a penetrating through-hole results in a decrease in the planarity or flatness of the ceramic green sheet and the difficulty in peeling of the substrate film is increased so much by the adhesive electro-conductive paste filling the through-holes.
The above mentioned troubles taking place in the ceramic green sheet by the removal of the substrate film lead to a drawback such as displacement of the ceramic green sheets under lamination. This problem is so serious, especially, when the chip size is small resulting in a decrease of or even total loss of the performance as an inductor device.
It is accordingly desired to develop a method by which the substrate film can be removed from a ceramic green sheet having a so small thickness of 30 to 20 xcexcm or even smaller without troubles such as breaking, crease formation and stretching of or in the ceramic green sheet.
Several methods are proposed heretofore to overcome the above mentioned difficulties in the removal of the substrate film from a ceramic green sheet or, in particular, ferrite green sheet of a so small thickness. Instead of laying two ceramic green sheets each freed from the substrate film, for example, ceramic green sheets each lined with a substrate film are laminated as such followed by removal of the substrate film. Thereafter, another ceramic green sheet lined with a substrate film is laid as such onto the lamination of ceramic green sheets to be press-bonded for further lamination. This procedure is repeated as many times as desired until a necessary number of laminated ceramic layers can be accomplished (Japanese Patent Kokai 5-36568 and 7-192955). This method is referred to hereinafter as the leaf bonding/peeling lamination method.
This leaf bonding/peeling lamination method is of course advantageously applied to the preparation of a laminated inductor device from ferrite green sheets having a thickness of 20 xcexcm or smaller. A problem in the application of this method is that press-bonding and peeling of the substrate film must be repeated many times in lamination of every ferrite green sheet and, as the thickness of the ferrite green sheet is decreased, the peeling work of the substrate film must be performed with utmost care in order to avoid troubles in the ferrite green sheets of a so small thickness so that the working efficiency cannot be high enough requiring a long working time.
It is important accordingly to improve the working efficiency by decreasing the working time for peeling of the substrate film off the ceramic green sheet without occurrence of troubles in the ceramic green sheets such as breaking, crease formation and stretching.
The present invention accordingly has an object, in the preparation of a laminated inductor device by laminating a plurality of ceramic or ferrite green sheets of small dimensions having a thickness of 30 xcexcm or smaller, to provide a laminated inductor device of high quality in high working efficiency without occurrence of troubles in the ferrite green sheets such as breaking, crease formation and stretching in the course of the peeling work of the substrate film from the ferrite green sheet.
Thus, the present invention provides, in a method for the preparation of a laminated ferrite inductor device comprising the steps of (a) forming a through-hole to penetrate a first laminated body consisting of a first substrate film and a first ferrite green sheet, (b) forming a coil pattern on the ferrite green sheet, (c) cutting the laminated body provided with a through-hole and a coil pattern into card-formed sheets, (d) removing the substrate film from the ferrite green sheet to give an unsupported ferrite green sheet as a base, (e) laying a second card-formed sheet on the base with the ferrite green sheet of the second card-formed sheet in contact with the base, (f) press-bonding the base and the second card-formed sheet, (g) removing the substrate film of the second card-formed sheet by peeling, (h) repeating the steps (a) to (g) to form a laminated block of ferrite green sheets and (i) sintering the laminated block of ferrite green sheets, the improvement which comprises forming, in addition to the coil pattern, a peel-facilitating pattern on the ferrite green sheet forming the laminated body with the substrate film and forming a snap groove in the ferrite green sheet along the outer periphery of the peel-facilitating pattern.
It is preferable that the above mentioned snap groove formed in the ferrite green sheet has a depth not smaller than the thickness of the ferrite green sheet to fully divide the ferrite green sheet but smaller than the total thickness of the ferrite green sheet and the substrate film so as not to cut apart the substrate film.
Further, the above mentioned peel-facilitating pattern is formed on the ferrite green sheet preferably by using an electroconductive printing paste in a line-formed pattern along the inward periphery of the snap groove to have a width of 1 to 2 mm and a thickness of 10 to 20 xcexcm.